Ok, things are ok now. Usertracks seem to load fine, even the ones I couldn't load before. Super strange.
Honestly, I don't know what I did.
I just created a new song instead of starting from an existing one.
Then I closed BIAB without doing anything (I had the feeling that BIAB had to save some settings by closing normally, without crashing. It probably makes no sense but ... I did).
Opened the program again, tried to load a UT, it was fine.
Tried again, fine again.
Finally I opened the old song in which I couldn't open any UT, and again now UTs load.
Very very strange.
But it works now smile